Are We Nearing a Planetary Boundary?

Josh Haner/The New York Times Reddish and brownish trees in this forest in Montana have been killed by the mountain pine beetle. The beetle population has soared in part because temperatures no longer plunge as low in the winter, so fewer get killed off. The earth could be nearing a point at which sweeping environmental changes, possibly including mass extinctions, would undermine human welfare, 22 prominent biologists and ecologists warned on Wednesday. Acknowledging in a new paper that both the likelihood and timing of such a planetary “state shift” were uncertain, the scientists nonetheless described warning signs that it could arrive within a few human generations, if not sooner. The problems are familiar by now: they include a planetary warming that, while slow on the scale of a human lifetime, is extremely rapid on a geologic time scale, the scientists said. And huma...